This application claims the priority of German patent document 10 2004 010 150.7, filed Feb. 27, 2004 (PCT International Application No. PCT/DE2005/000317, filed Feb. 25, 2005), the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a high-frequency MEMS switch having a bent switching element.
MEMS switches or switching elements in the MEMS technology (MEMS=Micro Electro Mechanical Systems) are used in many different fields, such as automobile electronics, telecommunications, medical engineering or measuring technology. As a result of their miniaturization, such switching elements further developed as a micro electro mechanical system are particularly suitable also for space flight applications and satellite systems. High-frequency MEMS switches are also particularly suited for use in radar systems, satellite communications systems, wireless communication systems and instrument systems. High-frequency MEMS switches are, for example, also required in phase antenna facilities and in the case of phase shifters for satellite-based radar systems.
High-frequency MEMS switches offer a number of advantages, such as an extremely low power consumption, good insulation or low interference capacities, low insertion loss or low insertion attenuations and low manufacturing costs.
The article “RF MEMS Switches, Switch Circuits and Phase Shifters” by Gabriel M. Rebeiz et al. in Revue HF No. 2/2001, describes MEMS switches which are used in the high-frequency range, in a range of between 0.1 and 100 GHz. These MEMS switches have cantilever switching arms in the form of mechanical springs which are operated by the effect of electrostatic force for the opening or closing of an electric circuit. The cantilever switching arm or cantilever bar is fastened on a substrate and is electrostatically attracted by an electrode in order to close a contact. Without an applied voltage, the switching arm returns into its starting position as a result of elastic restoring forces, and the contact is opened.
In the case of MEMS switches, the switching operation can be caused in different manners which are basically illustrated as examples in FIGS. 3a-f. In this case, a switching element influences the traveling of an electromagnetic wave on a signal line by opening or closing a transmission path. This can take place in the manner of a series-parallel switch, of a shunt switch or of a series-shunt switch. In the opened condition of the switching element, a large distance to the contact area is generally necessary because, in this condition, the capacitance should be as low as possible in order to obtain an interference-free line. However, a short distance is required for the switching operation itself since only low electrostatic forces are active.
The article by C. Chang and P. Chang “Innovative Micromachined Microwave Switch with Very Low Insertion Loss”, Proceedings of the 10th International Conference on Solid-State Sensor Actuators (Transducers 99), Jun. 7-10, 1999, Sendai, Japan, Page 1830-33, describes a MEMS switch with a bent switching element in the shape of a cantilever bar as a cantilever element. The switching element is fastened above a ground electrode with one end on a substrate, the remaining area of the switching element being oriented upward in a curved manner and projecting away from the substrate. When a switching voltage is applied, the upward-bent switching element is applied to the ground electrode by electrostatic forces, so that the free end of the switching element comes in contact with a signal line. Without the applied switching voltage, the switching element is moved back by an elastic tensile stress into the upward-oriented position in which it is far away from the signal line. During the back-and-forth switching between the two switching conditions, the switching element moves like a frog's tongue.
MEMS switches generally have the problem that the elastic restoring forces as a rule are very low, so that there is the danger that the switching element clings to the surface of the signal line as a result of adhesion. The switching elements therefore often lack sufficient reliability which is necessary for long-term missions, for example, in space.
It was therefore attempted to provide the switching element with a stronger design in order to achieve stronger restoring forces. However, the electrostatic forces are not sufficient in most cases for reliably causing the switching operations.
It is therefore an object of the present invention to provide a high-frequency MEMS switch having a bent switching element, which ensures a high long-term reliability while the interference capacities are low.
Another object of the invention is to provide such a switch in which a higher mechanical stability.
Finally, still another object is to provide a switch which achieves a greater switching force are achieved while the space requirement is low.
These and other objects and advantages are achieved by the high-frequency MEMS switch according to the present invention, which comprises a signal conductor arranged on a substrate. An oblong-shaped switching element has a bent elastic bending area and is fastened on the substrate in a cantilevered manner. An electrode arrangement generates an electrostatic force that acts upon the switching element, in order to bend it toward the signal conductor. The switching element in its longitudinal direction is arranged parallel to the signal conductor and has a contact area extending transversely to the switching element partially or completely over the signal conductor. Under the effect of electrostatic force, the elastic bending area of the switching element approaches the electrode arrangement parallel to the signal line in a progressive manner.
In the high-frequency MEMS switch according to the invention, the voltage required for closing the element is kept low, while a large switching path is permitted, so that the distance to the open condition is large and the capacitance is therefore low. By arranging the switching element in its longitudinal direction parallel to the signal conductor, a further miniaturization is also achieved, in which case the switching element can nevertheless have a relatively long design, and a higher mechanical stability and a greater switching force can therefore be achieved. In particular, a greater restoring force or a stronger switching element also become possible. As a result of the large possible length and surface of the switching element, greater electrostatic forces, on the one hand, and greater restoring forces or a thicker switching element, on the other hand, can be achieved.
The switching element preferably comprises at least two switching arms with a bent elastic bending area, which are arranged on both sides of the signal conductor and extend in their longitudinal direction parallel to the signal conductor. The switching arms are connected with one another by a bridge positioned over the signal conductor, which bridge is formed by the respective contact area. The reliability of the MEMS switch is even further increased because still higher restoring forces and electrostatic forces can be achieved while the space and energy demand is low. As a result, a particularly high mechanical stability and switching force are achieved while the space and energy requirements are low.
The electrode arrangement is advantageously formed by at least one ground or base electrode which is arranged below the switching element in a flat manner on the substrate in order to electrostatically attract the switching element. If the switching arms are arranged on both sides, the base electrode or ground electrode is arranged below each switching arm.
According to another preferred embodiment, the electrode arrangement is formed by a ground electrode arranged below the substrate or by the substrate itself. This results in a simplified production and therefore in reduced production costs. The substrate may be manufactured from high-ohmic silicon.
The electrode arrangement advantageously extends parallel to the substrate surface so that the electrostatic force pulls the switching element in its bending area progressively to the substrate surface. The bent bending area is preferably formed by bimorphic material.
Another advantageous further embodiment provides that, for generating a tensile stress, the bending area has a surface melted-on, for example, by laser heating. This has the advantage that the tensile stress can be adjusted by the corresponding selection of the duration and intensity of the laser irradiation corresponding to the respective demands. The tensile stress can also be achieved by the appropriate control of the layer deposition during production.
The switching element is advantageously produced by means of the thin-film technology. As a result, a cost-effective production and a small construction are achieved.
The contact area of the switching element preferably comes in direct contact with the signal conductor under the effect of the electrostatic force. As an alternative, under the effect of the electrostatic force, the contact area takes up a minimal distance from the signal conductor; that is, it does not come in direct contact with the signal conductor. This results in a high capacitance between the signal conductor and the switching element, so that the signal line is interrupted. The minimal distance can be achieved or maintained, for example, by a suitable dielectric insulation.
A method of producing a high-frequency MEMS switch having a belt switching element according to the invention includes the following steps: constructing a signal line on a substrate; as required, forming an electrode arrangement on the substrate (for example, if the substrate has no intrinsic conduction); forming an oblong switching element having a bent elastic bending area on the substrate such that, in its bending area, it is pulled by the electrode arrangement by an electrostatic force lengthwise toward the substrate and, by an elastic restoring force, in the bending area, moves away from the substrate. The switching element in its longitudinal direction parallel to the signal conductor is arranged such that a laterally projecting contact area of a the switching element extends transversely-over the signal conductor, so that the elastic bending area of the switching element, under the effect of the electrostatic force parallel to the signal line, progressively approaches the electrode arrangement in order to bring the contact area in the proximity of the signal conductor. The electrode arrangement may also be formed by an intrinsically conducting substrate or an intrinsically conducting substrate area.
By means of the method, a particularly reliable high-frequency MEMS switch having a bent switching element is produced in a cost-effective manner, which has an increased mechanical stability and higher switching forces.
Advantageously, the switching element is shaped such that it has at least two switching arms having a bent elastic bending area. The switching arms are arranged on both sides of the signal conductor, so that they extend in their longitudinal direction parallel to the signal conductor, and the switching arms are connected with one another by a bridge positioned over the signal conductor, which bridge is formed by the respective contact area.
Preferably, at least one base electrode as the electrode arrangement under the switching element is arranged flatly on the substrate. At least one ground electrode arranged below the substrate can also be formed as the electrode arrangement. Advantageously, the bending area is formed by bimorphic material. However, it is particularly advantageous for the surface of the bending area to be melted on by means of laser heating for generating a tensile stress. In particular, the method can be used for producing the high-frequency MEMS switch further developed according to the invention, as it is generally described above.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.